1. Field of the Invention
This invention relates to a self-controlled polyphase synchronous motor drive system in which current applied to a polyphase stator is provided by a rectifier-inverter or cycloconverter power converter with input power obtained from a fixed or variable frequency single or polyphase alternating current supply. More particularly, this invention relates to a self-controlled polyphase synchronous motor drive system wherein the frequency and phase of the current applied to the stator of a synchronous motor are derived from the frequency and phase of the polyphase stator voltage and the speed and position of the rotor relative to the stator. A phase control means responsive to advanced or delayed analog signals representing the polyphase stator voltage converts the analog voltages to voltage digital timing signals. The voltage digital timing signals are combined with rotor digital timing signals representative of the position of the rotor relative to the stator by a digital gating command signal generating means. The digital gating command signals are applied to control the timing of the converter thyristor gate signals and the current applied to the stator.
This invention also relates to a digital control system for removing distortion from the gate signals applied to the solid state controlled devices for controlling current applied to the stator. In addition, this invention relates to the sensing and control of the converter commutation margin in both the motoring and regenerative braking mode.
2. Description of the Prior Art
Self-controlled synchronous motor drive systems are known which utilize a static switch timing which is responsive to both the phase of machine voltage and the magnitude of the current to maintain reliable commutation under load. In such systems, a discrete transition or step occurs in transferring commutation from a position sensor to a voltage timing control at a particular synchronous motor speed. It is also known in the art to utilize synchronous motor voltage timing control of transistor switches in a brushless drive to effect commutation and self-control of a synchronous motor.
The use of motor power factor control and advancement of the same in response to the synchronous motor speed by use of a variable voltage threshold comparator responsive to the synchronous motor voltage signal which results in a discrete transition from a position sensor to a terminal voltage control circuitry is disclosed in U.S. Pat. No. 3,894,277. A self-controlled synchronous motor drive circuitry employing a voltage source converter wherein the phase of the synchronous motor current is sensed and used to control the phase of the applied voltage is disclosed in U.S. Pat. No. 3,997,825. In U.S. Pat. No. 3,869,099, issued to the inventor herein, the use of a position sensor timing signal and terminal voltage timing signal in a synchronous motor drive system is disclosed.
It is also known in the art to utilize a phase shifter circuit for controlling the power factor of a cycloconverter by generating reference signals for independently controlling the real power and reactive power of a self-controlled cycloconverter as described in U.S. Pat. No. 3,982,168.
In each of the known prior art self-controlled synchronous motor drive systems, the generation of digital timing signals which are responsive to a rotor position analog polyphase signal and an analog stator voltage signal to maintain a three phase current which is applied to a stator winding of a controlled synchronous motor to maintain three phase current commutation under load at all synchronous motor speeds is disclosed or suggested in the prior art.
It is also known in the art to detect distortion in the stator voltage and to replace the same with a selected voltage. The circuit disclosed in Kaeser, et al., U.S. Pat. No. 3,777,250, is typical.
In each of the known prior art self-controlled synchronous motor drive systems, the continuous blending of digital signals advanced or retarded from the stator voltage with digital signals representing rotor position, wherein the digital rotor position signals serve to blank the noise pulses appearing in the digital stator voltage derived signals, is not disclosed or suggested in the prior art. Also not disclosed or suggested in the prior art is the use of a commutation margin sensing and control circuit to assure reliable machine or input line current commutation in motoring or braking.